Swept quartz generally refers to quartz treated by a method such as described in U.S. Pat. No. 3,932,777 issued to J. C. King on Jan. 13, 1976. This method comprises subjecting a quartz crystal to an electric field at an elevated temperature.
Briefly, it has been known heretofore that the presence of such impurities as sodium ions in alpha quartz is detrimental to the properties of crystal resonators made from the quartz. In order to "sweep" the quartz material substantially free of these impurities, it has been the practice to heat the material to somewhat below 573.degree. C., the temperature at which the transition from alpha to beta quartz occurs. Then, a strong DC electric field, which typically ranges fron 500 volts per cm to six kilovolts per cm, has been applied substantially along the direction of the Z crystallographic axis of the quartz, and left on for several days. The field is kept on while the crystal cools to room temperature. The impurity ions are swept toward the cathode electrode. At the end of the process, a portion of the quartz contiguous to the cathode is removed and discarded and the remaining material is used to make crystal resonators that are substantially insensitive to the effects of radiation in the form of x-ray, gamma ray, electrons, etc.
Though the "sweeping" process has resulted in quartz that is relatively free from impurities, it would be desirable to "sweep" quartz to an even higher degree of purity in a shorter time.